In the case of a slice that looks like this 

FFFF@,,,F
F,,,,,,,F
F,,,,,,,F
FFFF,,,,F
F@,,,,,,F

This slice could correctly fit with a slice that looked like this

F@,,,,,,F
F,,,,,,,F
F,,FFF,,F
F,,FFF,,F
F,,,,,,,F
F@,,,,,,F

To look like this (added second slice on the top and bottom of the first slice):


   F@,,,,,,F
   F,,,,,,,F
   F,,FFF,,F
   F,,FFF,,F
   F,,,,,,,F
   F@,,,,,,F
FFFF@,,,FFFF
F,,,,,,,F
F,,,,,,,F
FFFF,,,,F
F@,,,,,,F
F@,,,,,,F
F,,,,,,,F
F,,FFF,,F
F,,FFF,,F
F,,,,,,,F
F@,,,,,,F


But it would not work if the second slice looked like this:

F@,,F
F,,,F
F,F,F
F,,,F
@,,,F


Which would produce the following:

    F@,,F
    F,,,F
    F,F,F
    F,,,F
    @,,,F
FFFF@,,,F
F,,,,,,,F
F,,,,,,,F
FFFF,,,,F
F@,,,,,,F  <----- here is a blocked passageway
F@,,FFFFF
F,,,F
F,F,F
F,,,F
@,,,F

As you can see, there is a 1-block high passage on two otherwise functional slices. To avoid this, ensure that the slice guide marker has 1 block clearance of open terrain below or above in all vertical segments.

In the above example, changing the first slice to this will fix the issue:

FFFF@,,,F
F,,,,,,,F
F,,,,,,,F
FFFF,,,,F
F,,,,,,,F <---- added this extra space, slice guide isn't vertically crowded
F@,,,,,,F